1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
Laser image forming apparatuses have been widely used for electrophotographic image recording. Such an image forming apparatus includes an optical scanning device, and a method of rotating a drum to form a latent image, while scanning laser beams, using a polygon scanner (for example, a polygon mirror) in an axial direction of the drum having photosensitivity has been generally used. In the field of electrophotography, image forming apparatuses are required to form an image with high write density at a high speed to improve image quality and operability.
As one method of realizing both high density and high speed, it can be considered to rotate a polygon scanner at a high speed. In this method, however, noise and vibration in the polygon scanner increase, power consumption also increases, and durability is deteriorated.
As another method of realizing both high density and high speed, it can be considered to use multi beams of light emitted from a light source.
As one example of a multi-beam lighting system, a lighting system in which a plurality of semiconductor lasers and coupling lenses provided in a pair with each semiconductor laser are arranged in a main scanning direction, and these are integrally supported has been proposed. This lighting system can decrease the size of a deflection-reflecting surface by crossing light beams in the main scanning direction near the deflection-reflecting surface of a deflector that deflects the beams from semiconductor lasers, and can reduce a difference in optical performance between the beams, because the beams after being deflected can be made to pass through substantially the same optical path of an imaging optical system. Such a multi-beam lighting system is also referred to as a “crossing multi-beam light source”. Accordingly, because a cheap semiconductor laser is used, and fewer components are used, a very cheap multi-beam lighting system and optical scanning device can be provided.
For example, Japanese Patent Application Laid-open No. H11-340570 discloses a photoelectric converter, which includes on the same substrate a plurality of photoelectric conversion units including a plurality of laminated semiconductor layers and having a side at an end in a direction perpendicular to the lamination direction.
Japanese Patent Application Laid-open No. H11-354888 discloses a semiconductor light emitting diode including an arrangement substrate, one substrate supported by the arrangement substrate, a plurality of light emitting units each including a plurality of semiconductor layers formed by being laminated on the side of the substrate opposite the arrangement substrate, a plurality of ohmic electrodes provided on the side of the light emitting units opposite the substrate and electrically connected in one-to-one correspondence with each light emitting unit, and at least one heat release layer formed on the side of respective light emitting units opposite the substrate by being laminated on the substrate via the light emitting units.
However, to use the photoelectric converter disclosed in Japanese Patent Application Laid-open No. H11-340570 and the semiconductor light emitting diode disclosed Japanese Patent Application Laid-open No. H11-354888 for the multi-beam light source, the power consumption is large and a heat output is large. Therefore, the upper limit of the number of beams is about four beams or eight beams in view of the cost. Further, because each element has one-dimensional arrangement of the light emitting units, if the number of beams is considerably increased, a deviation amount of the beams from an optical axis of an optical system increases, and beam characteristic can be deteriorated.
Therefore, a vertical cavity surface emitting laser (VCSEL) has been proposed, in which a plurality of light emitting points can be easily formed on one element two-dimensionally, and the power consumption is small.
Japanese Patent No. 3227226 discloses an image forming apparatus, which uses an optical scanning device including a light source that emits independently modulatable three or more laser beams from a light emitting unit arranged two-dimensionally, a laser beam deflector that periodically deflects the laser beams in respective directions, and a scanning optical system that images the laser beams on a surface to be scanned. In the optical scanning device, if it is assumed that the number of the laser beams is n, number of imaging spots formed on an image carrier are S1 to Sn from the top in a direction perpendicular to a scanning direction, and a distance from the imaging spot S1 to the imaging spots S2 to Sn respectively measured at right angles to the scanning direction is L2 to Ln, values D2 to Dn respectively obtained by dividing L2 to Ln by a scanning line pitch p are approximately integral numbers, and surpluses M2 to Mn as a result of dividing D2 to Dn by n are natural numbers less than n, which are different from each other.
Japanese Patent Application Laid-open No. 2004-287292 discloses an optical scanning device having a first substrate fitted to a housing fixed to an image forming apparatus body, on which a light emitting diode and a driving circuit therefor are mounted, a second substrate fitted to the housing away from the first substrate, on which a connector for connecting a harness from the image forming apparatus body is mounted, and an elastically deformable connection member that electrically connects a first substrate terminal with a second substrate terminal.
Japanese Patent Application Laid-open No. 2005-250319 discloses a lighting system formed by combining a plurality of sets of a light source in which a plurality of independently modulatable light emitting points are arranged two-dimensionally and a coupling lens that couples divergent beams emitted from the light source, an optical scanning device with the lighting system, and an image forming apparatus with the optical scanning device.
Generally, in the optical scanning device, to suppress occurrence of density unevenness on an image due to a change in a quantity of light with temperature fluctuation and change with lapse of time, auto power control (APC) is executed, by which the quantity of light emitted from the light source is monitored by a detector such as a photodiode, and an output level is controlled based on the result. In this case, with an edge emitting laser, the beams are emitted in two directions forward and backward. Therefore, if the beam emitted forward is used for scanning and the beam emitted backward is used for monitoring, even if the beam for scanning is returned to the light source, there is little influence on the monitoring result. However, with the VCSEL, because the beams are emitted only in one direction, the emitted beams need to be divided or diverged, to use one for scanning and the other for monitoring. In this case, return light to the light source can affect the monitoring result.
Recently, an optical scanning device using an overfilled optical system has been proposed, in which a width of a deflector surface of a deflector that deflects a plurality of light beams is smaller than an irradiation range of a plurality of light beams. Such an optical scanning device is disclosed in, for example, Japanese Patent Application Laid-open No. 2003-270577. By the overfilled optical system, the width of the deflector surface can be reduced, and therefore more deflector surfaces can be provided than in the conventional deflector having the same diameter (diameter of a circle centering on a rotation axis, with which the deflector touches internally). Therefore, the scanning speed can be improved without increasing the number of revolution of the deflector.
In the optical scanning device using the overfilled optical system, however, there is a problem in that light quantity distribution of the light beams reflected by the deflector surfaces is not uniform. Further, in the VCSEL arrays, because a divergence angle of the light beam is small, the light quantity distribution of the light beams incident to the deflector surfaces is slightly non-uniform as compared with that of the edge emitting laser. Therefore, to combine the vertical-cavity surface emitting type light source with the overfilled optical system, a technique for equalizing the light quantity distribution of the light beams is required.